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16 2                           TEETH   OF   EDENTATES                          CHAP.
inasmuch as many Edentates have teeth. It is, however, by
a number of small tooth - characters that the order can be
defined. Thus if teeth are present they are simple in struc-
ture, without enamel in the adult condition, though a rudi-
mentary enamel-organ has been discovered in an Armadillo.
The teeth, moreover, are not found in the anterior part of the
mouth, and they grow from persistent pulps; neither is there
much differentiation among them. It is not possible, however,
to speak of the Edentates as quite homodont, since in Orycteropus
there are large cheek-teeth ; but there is at any rate not a marked
heterodonty in that or in any other Edentate. It used to be said
that the Edentates were monophyodont. But the Armadillo
Tatusia was subsequently found to possess a second suppressed
dentition, and after this discovery Mr. Thomas proved that
Orycteropus is also diphyodont. Since then other Armadillos
have been shown to be diphyodont; and the whole group there-
fore, so far as concerns those members that have teeth, may in all
probability be regarded as typically mammalian in this respect.
These characters are slender enough, but there seem to be
no others by means of which the members of this order can. be
satisfactorily linked together. The fact is, that we have here
a polymorphic order which contains in all probability repre-
sentatives of at least two separate orders. We have at present
a very few, and these perhaps highly modified, descendants of a
large and diverse group of mammals. For convenience* sake they
will be all treated of under the head of Edentata.
Although for the probable reasons already stated it is a
hard matter to frame such a definition as will include all
existing Edentates, it is easy enough to define two groups in this
heterogeneous order; to define one group we ^should say, rather,
and then to regard the leavings as forming another not so easily
definable a group.
The perfectly-definable group is that which includes the
American Anteaters, the Armadillos, and the Sloths. In all
these creatures, which may certainly be regarded as representing
on their own account as many family types, there are a number
of important and highly-characteristic anatomical features which
they share in common. So exceedingly different are those three
types in general appearance and (correlated with that) in way of
life that these common characters acquire increased importance.